Air ducts are one of the most important factors in a building's ventilation and overall HVAC system. They carry and remove air to help meet room-by-room calculated heating and cooling loads. They are the main vehicle for ventilation air; the bridge to having proper indoor air quality and a comfortable indoor temperature. Correct ductwork installation is important to buildings' air quality and energy efficiency.
Correct duct installation will ensure air is flowing to where it is supposed to
Successfully installing and running sealed ductwork increases HVAC system efficiency and prevents costly repairs further down the road. Poorly installed ductwork can account for up to 30 percent of heating and cooling costs, costing hundreds if not thousands of dollars, depending on the building size. Other problems arising from poor ductwork installation can include uneven heating and substantial noise levels. Furthermore, poorly installed ducts can leak and then contaminate indoor air with mold, dust and toxins that pose a serious health hazard.
Pre-installation
Before installation, certain considerations must be undertaken. Nick Kaufmann, LEED AP BD&C, director of manufacturing and engineering, Duct-Sox Corp., Peosta, Iowa, believes that in spite of the mechanical requirements of ductwork installation, one of its first steps is simply understanding the customer’s and the building’s needs. “Some facilities request high-velocity systems, while others may have velocity-sensitive equipment,” he says. “The majority of clients are just looking for an even dispersion of air. Secondly, one will need to see if duct condensation is an issue. If it is, a slightly porous fabric ducting is required (a cost-effective and lightweight alternative to insulated duct). Once the airflow needs are identified and the required ducting is chosen, it is then just a matter of getting the air to the desired space.”
What’s unique about air duct installation as it applies to metal buildings? “One of the unique things about a metal building is the location of ceiling points to attach your ducting,” Kaufmann says. “Sometimes a metal building can go 20 to 30 feet without ceiling places to attach. In those cases, usually a wall-to-wall cable hanging system is utilized. Unit placement can also be affected. Can the building support a roof top unit or not?”
Check local regulations and codes concerning ducting systems, air supply and spacing regulations. Install duct equipment and procedures using guidelines from the Uniform Mechanical Code, the Air Diffusion Council, SMACNA and manufacturers’ specifications.
Establishing a routing plan for ductwork is important for correct venting. Extend the ducts to full length for proper sizing. Do not install when they are compressed, and don’t use longer lengths than needed. During this preplanning, be aware of sharp corners; duct bends should not be made across them if possible. A 90-degree elbow reportedly creates the same resistance as adding 5 feet of pipe. When turns are necessary, “duct elbows” should be used.
Duct paths must account for locations and directions of joists, roof hips, firewalls and other potential obstruction. There should be no incidental contact with metal fixtures, pipes or conduits that can compress or damage the ductwork. Avoid routing ducts under turbine vents, skylights, canopy windows, etc., where duct materials are exposed to direct sunlight, or near an ultraviolet (UV) light source. Also before installing, create enough space in the ductwork for cleaning and maintenance, construction and sealing and inspection.
Duct Support
Ducts need support. The most common support is 1 1/2-inch-minimum-width hanger/saddle material. Support intervals should be in accordance with the manufacturer’s recommendations, or as local codes specifies. Long duct runs with sharp bends may require additional supports before and after the bends. Don’t forget to screw the metal duct to the joist hanger so the ducts won’t rattle when someone walks across the floor above. Supports for flexible ducts should be spaced at no more than 4-foot intervals. Flexible ducts should be supported by strapping having a minimum width of 1 1/2 inches at all contact points with the duct. Supports should not constrict the inner liner of the duct. Flexible ducts should have maximum of 1/2-inch sag per foot between supports. Flexible ducts may rest on ceiling joists or truss supports as long as they lie flat and are supported at no more than 4-foot intervals. Vertically installed ducts also should be stabilized
Putting It Together
According to Stockton, Calif.-based ConSol, the following connection steps will ensure correct ductwork installation:
- All joints and seams of duct systems and their components should be sealed with mastic, a combination of mastic and embedded mesh, aluminum-backed butyl adhesive tape (15 mil. minimum), or pressure-sensitive tape approved for use by the duct manufacturer and meeting UL181 specifications. This excludes cloth-backed rubber-adhesive tapes (“approve tape”); cloth-backed rubber-adhesive tapes must not be used to attach or seal ducts.
- Junctions of collars to distribution boxes and plenums should be sealed with mastic.
- All sealants should be used in strict accordance with manufacturer’s installation instructions and within sealants moisture and temperature limitations.
- All tapes used as part of duct system installation should be applied to clean, dry surfaces and sealed with manufacturer’s recommended amount of pressure or heat. If oil is present, taped surfaces should be prepared with a cleaner/degreaser prior to application.
- It is recommended that all register boxes should be sealed to the drywall or floor with caulking or mastic.
- At least three equally-spaced #8 screws should be used to mechanically fasten round metal ducts (Three screws for ducts up to 12-inch diameter and five screws for ducts over 12-inch diameter).
- Square or rectangular metal ducts should be mechanically fastened with at least one screw per side.
- Flexible ducts should be joined by a metal sleeve, collar, coupling or coupling system. At least 2 inches of the beaded sleeve, collar, or coupling must extend into the inner core while allowing a 1-inch attachment area on the sleeve, collar or coupling for the application of tape.
- The flexible duct inner core should be mechanically fastened to all fittings, preferably using drawbands installed directly over the inner core and beaded fitting. If beaded sleeves and collars are not used, then the inner core should be fastened to the fitting using #8 screws equally spaced around the diameter of the duct, and installed to capture the wire coil of the inner liner.
- Again, these connections should be made in accordance with manufacturer’s specifications.
Learn More
To learn more about successful duct installation, Schaumburg, Ill.-based American Ductwork Council has produced the 5th Edition of the “Flexible Duct Performance & Installation Standards.” It’s a 28-page manual for use and reference by designers, architects, engineers, contractors, installers and users for evaluating, selecting, specifying and properly installing flexible duct in heating and air conditioning systems. Features covered in depth include: descriptions of typical styles, characteristics and requirements, testing, listing, reporting, certifying, packaging and product marking. Guidelines for proper installation are treated and illustrated in depth, featuring connections, splices and proper support methods for flexible duct. A single and uniform method of making end connections and splices is graphically presented for both non-metallic and metallic with plain ends.
